xbox series S shows unsupported mode after I changed the resolution

Just to confirm, are you trying to do the steps below in sequence?

If there's a disc in the console, eject it.

On the console, press and hold the Power button on the front of the console for 10 seconds to turn off the console.

Press and hold the Power button and the Eject button until you hear a beep to turn on the console. You’ll hear one beep right away and a second beep 10 seconds later. The power light will flash on before the second beep. Don’t let go until you hear the second beep or see the console start up in low resolution mode (the Xbox startup screen will appear slightly pixelated).

Note If you have an Xbox Series S or Xbox One S All-Digital Edition, press and hold the Xbox button  and the Pair button  instead, as your console doesn’t have an Eject button.

In either case, this will start up your console in low resolution mode (640 × 480).

switch console off and press and hold the sync button and power button, this should get you to the service menu so you can reset the resolution.

I understand, I would highly recommend trying it out with a different HDMI cable first or a different TV to see if it works.

Not yet as I don’t have another one at home, this has only happened since the big update for fortnight

Since you are using an Xbox Series S console, please press and hold the Xbox button and the Pair button instead when resetting the display settings.